Magnetic anisotropy and structural flexibility in the field-induced single ion magnets [Co{(OPPh)(EPPh)N}], E = S, Se, explored by experimental and computational methods.

During the last few years, a large number of mononuclear Co(II) complexes of various coordination geometries have been explored as potential single ion magnets (SIMs). In the work presented herein, the Co(II) S = 3/2 tetrahedral [Co{(OPPh)(EPPh)N}], E = S, Se, complexes (abbreviated as CoO2E2), bearing chalcogenated mixed donor-atom imidodiphosphinato ligands, were studied by both experimental and computational techniques. Specifically, direct current (DC) magnetometry provided estimations of their zero-field splitting (zfs) axial () and rhombic () parameter values, which were more accurately determined by a combination of far-infrared magnetic spectroscopy and high-frequency and -field EPR spectroscopy studies.

Effective coupling of rapid freeze-quench to high-frequency electron paramagnetic resonance.

We report an easy, efficient and reproducible way to prepare Rapid-Freeze-Quench samples in sub-millimeter capillaries and load these into the probe head of a 275 GHz Electron Paramagnetic Resonance spectrometer. Kinetic data obtained for the binding reaction of azide to myoglobin demonstrate the feasibility of the method for high-frequency EPR. Experiments on the same samples at 9.

Temperature-cycle electron paramagnetic resonance.

We report on a novel approach to the study of rates and short-lived intermediates of (bio)chemical reactions that involve paramagnetic species. Temperature-cycle Electron Paramagnetic Resonance (EPR) concerns the repeated heating of a reaction mixture in the cavity of an EPR spectrometer by pulsed irradiation with a near-infrared diode laser combined with intermittent characterization of the sample by 275 GHz EPR at a lower temperature at which the reaction does not proceed. The new technique is demonstrated for the reduction of TEMPOL with sodium dithionite in aqueous solution down to the sub-second time scale.

Analysis of the EPR spectra of transferrin: the importance of a zero-field-splitting distribution and 4-order terms.

Multi-frequency EPR spectroscopy can provide high-level structural information on high-spin Fe sites in proteins and enzymes. Unfortunately, analysis of the EPR spectra of these spin systems is hindered by the presence of broad distributions in the zero-field-splitting (ZFS) parameters, which reflect conformational heterogeneity of the iron sites. We present the analysis of EPR spectra of high-spin Fe bound to human serum transferrin.

Rapid Freeze-Quench EPR Spectroscopy: Improved Collection of Frozen Particles.

Rapid freeze-quench (RFQ) in combination with electron paramagnetic resonance (EPR) spectroscopy at X-band is a proven technique to trap and characterize paramagnetic intermediates of biochemical reactions. Preparation of suitable samples is still cumbersome, despite many attempts to remedy this problem, and limits the wide applicability of RFQ EPR. We present a method, which improves the collection of freeze-quench particles from isopentane and their packing in an EPR tube.

Temperature Determination by EPR at 275 GHz and the Detection of Temperature Jumps in Aqueous Samples.

Second-moment analysis along two dimensions of continuous-wave EPR spectra of nitroxides enables EPR thermometry in a broad temperature range. Simulations show that the temperature can be derived in both the slow-motion and the fast-motion regime, which is experimentally verified at 275 GHz for H2O/glycerol (50/50% by volume) and pure water. We demonstrate that this tool allows the calibration of temperature jumps induced by infrared laser irradiation of a submicroliter sample in the single-mode cavity of a 275 GHz spectrometer, which prepares for kinetic studies of processes involving paramagnetic species.

Simulation of multi-frequency EPR spectra for a distribution of the zero-field splitting.

We present a numerical procedure called 'grid-of-errors' to extract the distribution of magnetic interactions from continuous-wave electron-paramagnetic-resonance (EPR) spectra at multiple microwave frequencies. The approach is based on the analysis of the lineshape of the spectra and explicitly worked out for high-spin systems for which the lineshape is determined by a distribution of the zero-field splitting. Initial principal values of the zero-field splitting tensor are obtained from the EPR spectrum at a microwave frequency in the high-field limit, and the initial distribution is taken Gaussian.

Exploring the Fe(III) binding sites of human serum transferrin with EPR at 275 GHz.

We report 275 GHz EPR spectra of human serum transferrin. At this high microwave frequency the zero-field splitting between the magnetic sublevels of the high-spin [Formula: see text] sites can be accurately determined. We find the zero-field splitting to be a sensitive probe of the structure of the transferrin iron-binding sites.

Hydrogen bonding of nitroxide spin labels in membrane proteins.

On the basis of experiments at 275 GHz, we reconsider the dependence of the continuous-wave EPR spectra of nitroxide spin-labeled protein sites in sensory- and bacteriorhodopsin on the micro-environment. The high magnetic field provides the resolution necessary to disentangle the effects of hydrogen bonding and polarity. In the gxx region of the 275 GHz EPR spectrum, bands are resolved that derive from spin-label populations carrying no, one or two hydrogen bonds.

The type 1 copper site of pseudoazurin: axial and rhombic.

We report on a high-frequency electron-paramagnetic-resonance study of the type 1 copper site of pseudoazurin. The spectra fully resolve the contribution of a nearly axial spectrum besides the rhombic spectrum, which unequivocally proves the existence of two conformations of the copper site. Pseudoazurins have been considered from Achromobacter cycloclastes including eight mutants and from Alcaligenes faecalis.

A Mononuclear Mn(II) Pseudoclathrochelate Complex Studied by Multi-Frequency Electron-Paramagnetic-Resonance Spectroscopy.

Knowledge of the correlation between structural and spectroscopic properties of transition-metal complexes is essential to deepen the understanding of their role in catalysis, molecular magnetism, and biological inorganic chemistry. It provides topological and, sometimes, functional insight with respect to the active site properties of metalloproteins. The electronic structure of a high-spin mononuclear Mn(II) pseudoclathrochelate complex has been investigated by electron-paramagnetic-resonance (EPR) spectroscopy at 9.

Involvement of Tyr108 in the enzyme mechanism of the small laccase from Streptomyces coelicolor.

The enzyme mechanism of the multicopper oxidase (MCO) SLAC from Streptomyces coelicolor was investigated by structural (XRD), spectroscopic (optical, EPR), and kinetics (stopped-flow) experiments on variants in which residue Tyr108 had been replaced by Phe or Ala through site-directed mutagenesis. Contrary to the more common three-domain MCOs, a tyrosine in the two-domain SLAC is found to participate in the enzyme mechanism by providing an electron during oxygen reduction, giving rise to the temporary appearance of a tyrosyl radical. The relatively low k(cat)/K(M) of SLAC and the involvement of Y108 in the enzyme mechanism may reflect an adaptation to a milieu in which there is an imbalance between the available reducing and oxidizing co-substrates.

High-frequency EPR study of the high-spin FeII complex Fe[(SPPh2)2N]2.

We report continuous-wave electron-paramagnetic-resonance (EPR) spectra of the high-spin Fe(II) complex Fe[(SPPh(2))(2)N](2) at 275.7 GHz, 94.1 GHz and 9.

Multifrequency EPR study of Fe3+ and Co2+ in the active site of desulforedoxin.

The understanding of the electronic structure of S > 1/2 transition-metal sites that show a large zero-field splitting (ZFS) of the magnetic sublevels benefits greatly from study by electron-paramagnetic-resonance (EPR) spectroscopy at frequencies above the standard 9.5 GHz. However, high-frequency EPR spectroscopy is technically challenging and still developing.

A comment on the pseudo-nuclear Zeeman effect.

For high-spin systems whose magnetic sublevels are arranged in doublets at zero field, the electron-paramagnetic-resonance (EPR) spectra are commonly described by an effective spin Hamiltonian. We show that also in this approach, if the mixing of the electron spin states by the hyperfine interaction is negligible, a proper description of electron-nuclear double resonance (ENDOR) spectra can be obtained using a nuclear spin Hamiltonian in which the electron spin angular momentum operator is replaced by its expectation value. Appropriate values of this expectation value can be obtained from a wave function correct to first-order in the electron Zeeman interaction.

Theoretical analysis of the spin Hamiltonian parameters in Co(II)S4 complexes, using density functional theory and correlated ab initio methods.

A systematic Density Functional Theory (DFT) and multiconfigurational ab initio computational analysis of the Spin Hamiltonian (SH) parameters of tetracoordinate S = 3/2 Co((II))S(4)-containing complexes has been performed. The complexes under study bear either arylthiolato, ArS(-), or dithioimidodiphosphinato, [R(2)P(S)NP(S)R'(2)](-) ligands. These complexes were chosen because accurate structural and spectroscopic data are available, including extensive Electron Paramagnetic Resonance (EPR)/Electron Nuclear Double Resonance (ENDOR) studies.

Configuration of spheroidene in the photosynthetic reaction center of Rhodobacter sphaeroides : a comparison of wild-type and reconstituted R26.

We compare the resonance Raman spectra acquired at two excitation wavelengths, 496.5 and 514.5 nm, of spheroidene in the wild-type reaction center of Rhodobacter sphaeroides and reconstituted into the reaction center of the Rhodobacter sphaeroides mutant R26.

Continuous-wave EPR at 275GHz: application to high-spin Fe(3+) systems.

The 275GHz electron-paramagnetic-resonance spectrometer we reported on in 2004 has been equipped with a new probe head, which contains a cavity especially designed for operation in continuous-wave mode. The sensitivity and signal stability that is achieved with this new probe head is illustrated with 275GHz continuous-wave spectra of a 1mM frozen solution of the complex Fe(III)-ethylenediamine tetra-acetic acid and of 10mM frozen solutions of the protein rubredoxin, which contains Fe(3+) in its active site, from three different organisms. The high quality of the spectra of the rubredoxins allows the determination of the zero-field-splitting parameters with an accuracy of 0.

Stretched polyethylene films probed by single molecules.

Stretched films of low-density polyethylene (LDPE) doped with 2.3,8.9-dibenzanthanthrene (DBATT) were studied using polarization-selective single-molecule spectroscopy at 1.

A multifrequency high-field electron paramagnetic resonance study of Co(II)S(4) coordination.

Advanced electron paramagnetic resonance (EPR) methods have been employed in the study of two high-spin cobalt(II) complexes, Co[(SPPh(2))(2)N](2) (Co(Ph,Ph)L(2)) and Co[(SPPh(2))(SP(i)Pr(2))N](2) (Co(iPr,Ph)L(2)), in which the bidentate disulfidoimidodiphosphinato ligands make up for a pseudotetrahedral sulfur coordination of the transition metal. The CoS(4) core in the two complexes has slightly different structure, owing to the different peripheral groups (phenyl or isopropyl) bound to the phosphorus atoms. To determine the zero-field splitting, notoriously difficult for high-spin cobalt(II), the two complexes required different approaches.

A Proton ENDOR Study of Azurin.

As part of our ongoing project that aims at the optimum characterization of the electronic structure of the blue-copper site of azurin from Pseudomonas aeruginosa, we present the complete hyperfine tensors of the protons bound to the Cbeta atom of the copper-bound cysteine 112. These tensors have been obtained from a 95 GHz pulsed electron-nuclear double resonance study of a single crystal of the protein.

FID detection of EPR and ENDOR spectra at high microwave frequencies.

High-frequency pulsed EPR spectroscopy allows FID detection of EPR spectra owing to the short dead time that can be achieved. This FID detection is particularly attractive for EPR and ENDOR spectroscopy of paramagnetic species that exhibit inhomogeneously broadened EPR lines and short dephasing times. Experiments are reported for the metalloprotein azurin at 275 GHz.

Identification of a radical intermediate in the enzymatic reduction of oxygen by a small laccase.

The enzyme mechanism of the Cu-containing small laccase (SLAC) from Streptomyces coelicolor has been investigated by optical and electron paramagnetic resonance spectroscopy. A new intermediate was identified after the reaction of molecular oxygen with the reduced trinuclear site of the type-1-depleted (T1D) form of the enzyme. It has the fingerprint of a biradical with a triplet ground state.

A W-band pulsed EPR/ENDOR study of Co(II)S(4) coordination in the Co[(SPPh(2))(SP(i)Pr(2))N](2) complex.

Spin-echo detection at 95 GHz enables an electron-paramagnetic-resonance study of a cobalt complex with a bio-mimetic coordination of the transition metal by four sulfur atoms. A magnetically diluted single crystal of the complex has been investigated in great detail. Electron-nuclear double-resonance signals were observed of ligand nuclei and complete hyperfine tensors of the distinct phosphorus nuclei were derived, assigned and discussed.